An amplifier, also referred to as a trans impedance amplifier (TIA) circuit, is known, which converts an electric signal converted from an optical signal by a photodetector, such as a photodiode, into a voltage signal and amplifies the signal. For example, an amplifier is known, which converts a current signal converted from an optical signal into a voltage signal by using a common base or common gate transistor (e.g., see Patent Documents 1 to 3 and Non-Patent Document 1). Further, a differential amplifier is known, which takes a current signal converted from an optical signal to be one of input signals (e.g., see Patent Documents 4 and 5).